On Fri 2016-09-30 09:48:32, Sergey Senozhatsky wrote:
> On (09/29/16 13:28), Petr Mladek wrote:
> > On Wed 2016-09-28 10:18:45, Sergey Senozhatsky wrote:
> > > On (09/27/16 18:02), Petr Mladek wrote:
> > > > The main trick is that we replace the per-CPU function pointer
> > > > by a preempt_count-like variable that could track the printk context.
> > > >
> > > > I know that Sergey has another ideas in this area. But I wanted to see
> > > > how this approach would look like.
> > >
> > > well, yes. I was looking at WARN_*_DEFERRED [1] for some time, and, I
> > > think, the maintenance cost of that solution is just too high:
> > >
> > > a) every existing WARN_* in sched/timekeeping/who knows where else
> > > must be evaluated to ensure that in can't be called from printk()
> > > path. if `false' - then the corresponding macro must be replaced
> > > with _DEFERRED flavor.
> > >
> > > b) any patch that adds new WARN_* usages must be additionally checked
> > > to ensure that each of new WARN_* macros cannot be called from printk
> > > path. if `false' -- the corresponding macro must be replaced with
> > > _DEFERRED flavor.
> > >
> > > c) any patch that refactors the code or moves some function calls around
> > > etc. must be additionally checked for any accidental WARN_* from printk
> > > path. even though if none of the patches added any new WARN_* to the
> > > code.
> > >
> > > b) apart from WARN_* there can be `accidental' pr_err/pr_debug/etc. not
> > > necessarily newly added (see 'c').
> > >
> > >
> > > that's too much.
> > >
> > > it takes a lot of additional effort, because both reviewer and contributor
> > > must consider printk() internals. and, what's worse, if something goes
> > > unnoticed we end up having a printk() deadlock again.
> > >
> > > so I decided to address some of printk() issues in printk.c, not in
> > > kernel/time/timekeeping.c or kernel/sched/core.c or anywhere else.
I do not longer see how this might be achieved. If a printk()/WARN()
in the scheduler/timekeeping code can be reached from printk() then
it might too be reached outside printk. In this case, printk()
will not know about it and will happily call the scheduler/timekeeping
code recursively. This might still cause deadlock.
> > I see the point.
>
> well, just my 5 cents.
>
> > Your approach (alt buffer) adds some complexity to the printk code
>
> it does.
> the other thing is that there are several ways to deadlock printk().
> alt_printk is addressing deadlocks that were caused by printk()
> recursion only.
>
> printk()
> acquire_lock(&foo)
> printk()
> acquire_lock(&foo)
This looks theoretical. The recursion in printk() is not easily
possible at the moment. It is prevented by logbuf_cpu check when
logbug_lock is taken. It is prevented by console_trylock() when
console_sem is taken.
> which is a sub-set of all of the printk() deadlock scenarios. all of
> the locks that printk() acquires can be taken outside of printk() path.
>
> for example, cat /proc/console locks the console_lock() for seq output.
> thus we can have something like
>
> console_unlock() // lock &sem->lock
> up()
> activate_task()
> WARN_ON()
> printk()
> console_trylock() // lock &sem->lock
The WARN_ON() here is called under &p->pi_lock that is taken
by try_to_wake_up(). This WARN_ON() can be triggered also
outside printk()/console_unlock(). Therefore it needs to get
replaced by WARN_DEFERRED() anyway.
> DEFERRED_WARN is a good thing; it's just quite hard to keep everything
> working, given that any of those "9 patches per hour" can break something
> with just one WARN_ON().
>
>
> I assume that doing something like this
>
> #define WARN_ON(condition, format...) ({ \
> printk_deferred_enter(); \
> WARN(condition, ##format); \
> printk_deferred_exit(); \
> })
>
> is less than exciting because WARN_ON from irq won't immediately print
> the backtrace anymore.
Yup, we might need WARN_ON_DEFERRED() variant.
Best Regards,
Petr
